1. Field of the Invention
This invention relates to plastic lenses having a high refractive index of not less than 1.55 and excellent properties in visible light transmission in processability, impact resistance, and dyeability. It relates also to a method for manufacturing such lenses.
2. Description of the Prior Art
In recent years, plastic lenses have become widely used in optical instruments, since they are lighter in weight, less fragile and more readily colored in comparison with inorganic glass lenses. Especially, in the case of eyeglass lenses, light-weight lenses are desirable, because the total weight of a pair of lenses is important from both physiologic and optical viewpoints. This is one of the main reasons for the recent widespreading tendency of plastic lenses. The leading resin presently used for eyeglass lenses is a polymer of diethylene glycol bisallylcarbonate (commonly called CR-39, Trademark for PPG Industries Co.). Since the specific gravity of the above polymer (1.32) is smaller than that of inorganic glass lenses (2.54), it is possible to reduce to a great extent the weight of lenses when this polymer is used. The above polymer, however, has a refractive index of 1.50 which is smaller than that (n.sub.D =1.52) of inorganic glass lenses. As a consequence, it is necessary to increase the central thickness, peripheral thickness and curvature of the lenses made of this polymer, as compared with inorganic glass lenses. Accordingly, development of a transparent polymer having a higher refractive index has been eagerly awaited. Polycarbonate (n.sub.D =1.58) and polystyrene (n.sub.D =1.60) are known popularly as polymers having high refractive indices. Moreover, the use of these polymers in optical field is limited to comparatively small areas owing to their disadvantages such as unsatisfactory grinding processability, low scratch resistance due to insufficient surface hardness, and low resistance to organic solvents.
The proposed methods for providing three-dimensionally crosslinked plastic lens materials having high refractive indices include those which employ as major constituents (1) di(meth)acrylates derived from bisphenol-A [Japanese Patent Application "Kokai" (Laid-open) No. 116,301/83], (2) di(meth)acrylates derived from halogenated bisphenol-A [Japanese Patent Application "Kokai" (Laid-open) No. 10,491/82], (3) a combination of a halogenated styrene monomer and a polyfunctional methacrylate [Japanese Patent Application "Kokai" (Laid-open) Nos. 104,101/82, 28,118/82, and 28,116/82], and (4) a diallyl phthalate monomer [Japanese Patent Application "Kokai" (Laid-open) Nos. 212,401/82 and 15,513/83]. However, the method (1) provides with difficulty three-dimensionally crosslinked plastics having a refractive index of not less than 1.55. The methods (2), (3), and (4) provide three-dimensionally crosslinked plastics having a refractive index of not less than 1.55, but these plastics tend to discolor during the cast polymerization and are unsatisfactory also in weatherability and visible light transmission. In general, multifunctional (meth)acrylates cannot be purified by distillation; and, in addition, they contain polymerization inhibitors to improve the storage stability and are difficult to alleviate discoloration of the cured products caused by said inhibitors during the cast-polymerization.